Nanocrystalline semiconductors have attracted a considerable amount of attention due to their unique physiochemical properties and potential applications in novel optical, electrical, and optoelectrical devices. Recent advances in nanocrystals are having a dramatic impact on the development of next generation low-cost and/or high-efficiency solar cells. For example, Gur et al., reported air-stable, all-inorganic nanocrystal solar cells processed from solution using CdSe and CdTe nanorods. (Gur et al., Science, 310:462-465 (2005).) CuInSe2 and its related alloys, including CuInS2, CuGaSe2, and CuGaS2, are promising materials for photovoltaics due to their unique structural and electrical properties. Thin-film solar cells formed with these materials are highly stable against radiation, which makes them ideal for space applications. Semiconductor nanocrystal inks can be used to lower the fabrication cost of the absorber layers of the solar cells. In addition, hybrid organic and inorganic materials are promising for the realization of low-cost roll-to-roll printing of solar cells.
Semiconductors with a direct bandgap between 1 eV and 2 eV, including Cu(In,Ga)Se2 (1.04−1.6 eV) and CuIn(Se,S)2 (1.04-1.53 eV), are suitable for single-junction cells utilizing the visible spectrum. However, half of the solar energy available to the Earth lies in the infrared region. Inorganic quantum dot-based solar cells with a decent efficiency near 1.5 μm have been reported. Therefore, syntheses of narrow gap IV-VI (SnTe, PbS, PbSe, PbTe), II-IV (HgTe, CdxHg1-xTe), and III-V (InAs) QDs have attracted significant attention and these materials have potential uses for a variety of other optical, electronic, and optoelectronic applications. SnTe with an energy gap of 0.18 eV at 300K can be used for IR photodetectors, laser diodes, and thermophotovoltaic energy converters.
Conventionally, small-scale batch processes have been used to synthesize nanocrystals. However, agglomerated, amorphous nanoparticles are obtained, and high-temperature annealing may be required to achieve a desired crystalline structure. For the synthesis of size and shape-controlled nanocrystals, a hot injection method is more suitable. Murray et al. pioneered a hot injection method to synthesize various metal and semiconductor nanocrystals, particularly those having diverse compositions, sizes and shapes. (Murray et al., Annual Review of Materials Science, 30:545-610 (2000).) In a typical ‘hot injection’ synthesis, organic ligands are used to passivate the surface of the nanoparticles to prevent particle aggregation. The reactants are injected into a hot coordinating solvent for rapid nucleation and a controlled growth process. Moreover, nanoparticles with monodispersed sizes and shapes can be synthesized by controlling the concentration and functional group of the organic ligands.
The synthesis of CuInSe2 nanoparticles using the hot injection technique was first presented by Malik et al. in trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) by a two step reaction. (Malik et al., Advanced Materials 11:1441-4 (1999).) In this reaction, a TOP solution of CuCl and InCl3 was injected into TOPO at 100° C. and then followed by a hot injection of trioctylphosphine selenide (TOPSe) at an elevated temperature of 330° C. to initiate the nucleation and growth of nanoparticles. Spherical CuInSe2 nanoparticles of about 4.5 nm were synthesized according to the authors, and the powder X-Ray diffraction (“PXRD”) data presented indicated that binary materials such as Cu2Se and In2O3 were present as by-products.
However, these hot injection methods rely primary on batch procedures, and typically require long processing time (hours to days), inert conditions (Schlenk line and/or glove box), and long heating and cooling procedures. In addition, reaction conditions may be difficult to control in these batch processes, resulting in poor homogeneous nucleation and poor temperature control when attempting to scale up the procedure. Thus, a need exists for an apparatus and method that can provide continuous, scalable and rapid synthesis of size- and/or shape-controlled nanocrystals.